1. Field of the Invention
This invention is directed to coatings for implantable medical devices, such as drug eluting vascular stents.
2. Description of the State of the Art
Percutaneous transluminal coronary angioplasty (PTCA) is a procedure for treating heart disease. A catheter assembly having a balloon portion is introduced percutaneously into the cardiovascular system of a patient via the brachial or femoral artery. The catheter assembly is advanced through the coronary vasculature until the balloon portion is positioned across the occlusive lesion. Once in position across the lesion, the balloon is inflated to a predetermined size to radially compress against the atherosclerotic plaque of the lesion to remodel the lumen wall. The balloon is then deflated to a smaller profile to allow the catheter to be withdrawn from the patient's vasculature.
A problem associated with the above procedure includes formation of intimal flaps or torn arterial linings which can collapse and occlude the conduit after the balloon is deflated. Moreover, thrombosis and restenosis of the artery may develop over several months after the procedure, which may require another angioplasty procedure or a surgical by-pass operation. To reduce the partial or total occlusion of the artery by the collapse of arterial lining and to reduce the chance of the development of thrombosis and restenosis, a stent is implanted in the lumen to maintain the vascular patency.
Stents are used not only as a mechanical intervention but also as a vehicle for providing biological therapy. As a mechanical intervention, stents act as scaffoldings, functioning to physically hold open and, if desired, to expand the wall of the passageway. Typically, stents are capable of being compressed, so that they can be inserted through small vessels via catheters, and then expanded to a larger diameter once they are at the desired location. Examples in patent literature disclosing stents which have been applied in PTCA procedures include stents illustrated in U.S. Pat. No. 4,733,665 issued to Palmaz, U.S. Pat. No. 4,800,882 issued to Gianturco, and U.S. Pat. No. 4,886,062 issued to Wiktor.
Biological therapy can be achieved by medicating the stents. Medicated stents provide for the local administration of a therapeutic substance at the diseased site. In order to provide an efficacious concentration to the treated site, systemic administration of such medication often produces adverse or toxic side effects for the patient. Local delivery is a preferred method of treatment in that smaller total levels of medication are administered in comparison to systemic dosages, but are concentrated at a specific site. Local delivery thus produces fewer side effects and achieves more favorable results. One proposed method for medicating stents involves the use of a polymeric carrier coated onto the surface of a stent. A solution which includes a solvent, a polymer dissolved in the solvent, and a therapeutic substance dispersed in the blend is applied to the stent. The solvent is allowed to evaporate, leaving on the stent surface a coating of the polymer and the therapeutic substance impregnated in the polymer. Once the stent has been implanted at the treatment site, the therapeutic substance has a sustained release profile from the polymer.
Although local administration of therapeutic agents via stents has shown favorable results in reducing restenosis, improvements can be made to stent coatings. The controlled release of hydrophilic drugs, peptides, proteins, oligonucleotides, plasmids and DNA can be difficult with polymeric coatings due to the osmotic pressure from water absorption generated by the hydrophilicity of the agent. Upon penetration of water into the coating, the permeability of the coating for the hydrophilic drug is significantly increased, resulting in the elution of the drug at a therapeutically ineffective rate. Accordingly, it is desirable to provide a coating that includes a hydrophobic fraction to minimize water absorption. Moreover, hydrophobic polymers tend to adhere better to stent substrates and the mechanical integrity of high molecular weight hydrophobic polymers is typically superior. If the matrix is too hydrophobic, however, the polymer may have a poor solubility in solvent systems which are required to dissolve the drug to form a homogenous coating solution. The hydrophilic drug will rapidly phase separate in an uncontrolled manner resulting in drug aggregation. Drug aggregation produces unpredictable and variable release rate profiles. The result is a drug coating on a stent that is irreproducible such that each coated stent will have variable drug content and distribution with different release profiles. Particles of hydrophilic drugs also weaken a hydrophobic coating and cause local areas of high swelling. As a manufacturing stand point, replication of the same coating with similar drug distribution and release rate profile is desired.
A hydrophilic fraction in a coating is needed to make the agent more compatible in the coating, decrease the size of the drug aggregation, and minimize variability of the release rate of the drug from the coating. Hydrophilicity is important for both solution stability and the equal distribution of the drug in the coating. If, however, the coating is highly hydrophilic, the stent coating will rapidly absorb water. The rapid absorption of water plasticizes the polymer, resulting in a depression of the glass transition temperature to produce a soft gel-like coating. The mechanical properties of a plasticized gel-like reservoir are insufficient for wet expansion. The polymer can tear upon expansion and produce a coating with undesirable defects. Excessive water swelling not only weakens the polymer, but also increases the diffusivity of the drug, resulting in loss of release control.
The embodiments of the present invention address these concerns as well as others that are apparent by one having ordinary skill in the art.